Safe, efficient and economical operation of a motor vehicle depends, to a significant degree, on maintaining correct air pressure in all (each) of the tires of the motor vehicle. Operating the vehicle with low tire pressure may result in excessive tire wear, steering difficulties, poor road-handling, and poor gasoline mileage, all of which are exacerbated when the tire pressure goes to zero in the case of a “flat” tire.
The need to monitor tire pressure when the tire is in use is highlighted in the context of “run-flat” (driven deflated) tires, tires which are capable of being used in a completely deflated condition. Such run-flat tires, as disclosed for example in commonly-owned U.S. Pat. No. 5,368,082, incorporated in its entirety by reference herein, may incorporate reinforced sidewalls, mechanisms for securing the tire bead to the rim, and a non-pneumatic tire (donut) within the pneumatic tire to enable a driver to maintain control over the vehicle after a catastrophic pressure loss, and are evolving to the point where it is becoming less and less noticeable to the driver that the tire has become deflated. The broad purpose behind using run-flat tires is to enable a driver of a vehicle to continue driving on a deflated pneumatic tire for a limited distance (e.g., 50 miles, or 80 kilometers) prior to getting the tire repaired, rather than stopping on the side of the road to repair the deflated tire. Hence, it is generally desirable to provide a low tire pressure warning system with in the vehicle to alert (e.g., via a light or a buzzer) the driver to the loss of air pressure in a pneumatic tire.
To this end, a number of electronic devices and systems are known for monitoring the pressure of pneumatic tires, and providing the operator of the vehicle with either an indication of the current tire pressure or alerting the operator when the pressure has dropped below a predetermined threshold level.
For example, U.S. Pat. No. 4,578,992 (Galasko, et al; 04/86), incorporated in its entirety herein, discloses a tire pressure indicating device including a coil and a pressure-sensitive capacitor forming a passive oscillatory circuit having a natural resonant frequency which varies with tire pressure due to changes caused to the capacitance value of the capacitor. The circuit is energized by pulses supplied by a coil positioned outside the tire and secured to the vehicle, and the natural frequency of the passive oscillatory circuit is detected. The natural frequency of the coil/capacitor circuit is indicative of the pressure on the pressure-sensitive capacitor.
It is also known to monitor tire pressure with an electronic device which is not merely a passive resonant circuit, but rather is capable of transmitting a radio frequency (RF) signal indicative of the tire pressure to a remotely-located receiver. Such a “transmitting device” may have its own power supply and may be activated only when the pressure drops below a predetermined threshold. Alternatively, the transmitting device may be activated (“tured ON”) by an RF signal from the remotely-located receiver, in which case the receiver is considered to be an “interrogator”. Additionally, the transmitting device may be powered by an RF signal from the interrogator. Additionally, the electronic device which monitors the tire pressure may have the capability of receiving information from the interrogator, in which case the electronic device is referred to as a “transponder”.
As used herein, a “transponder” is an electronic device capable of receiving and transmitting radio frequency signals, and impressing variable information (data) in a suitable format upon the transmitted signal indicative of a measured condition (e.g., tire pressure) or conditions (e.g., tire pressure, temperature, revolutions), as well as optionally impressing fixed information (e.g., tire ID) on the transmitted signal, as well as optionally responding to information which may be present on the received signal. The typical condition of paramount interest for pneumatic tires is tire pressure. “Passive” transponders are transponders powered by the energy of a signal received from the interrogator. “Active” transponders are transponders having their own power supply (e.g., a battery), and include active transponders which remain in a “sleep” mode, using minimal power, until “woken up” by a signal from an interrogator, or by an internal periodic timer, or by an attached device. As used herein, the term “tag” refers either to a transponder having transmitting and receiving capability, or to a device that has only transmitting capability. Generally, tags which are transponders are preferred in the system of the present invention. As used herein, the term “tire-pressure monitoring system” (TPMS) indicates an overall system comprising tags within the tires and a receiver which may be an interrogator disposed within the vehicle.
It is known to mount a tag, and associated condition sensor (e.g., pressure sensor) within each tire of a vehicle, and to collect information from each of these transponders with a common single interrogator (or receiver), and to alert a driver of the vehicle to a low tire pressure condition requiring correction (e.g., replacing the tire). For example, U.S. Pat. No. 5,540,092 (Handfield, et al.; 1996), incorporated in its entirety by reference herein, discloses a system and method for monitoring a pneumatic tire. FIG. 1 therein illustrates a pneumatic tire monitoring system (20) comprising a transponder (22) and a receiving unit (24).
Examples of RF transponders suitable for installation in a pneumatic tire are disclosed in U.S. Pat. No. 5,451,959 (Schuermann; September 1995), U.S. Pat. No. 5,661,651 (Geschke, et al.; August 1997), and U.S. Pat. No. 5,581,023 (Handfield, et al.; December 1996), all incorporated in their entirety by reference herein. The described transponder systems include interrogation units, pressure sensors and/or temperature sensors associated with the transponder, and various techniques for establishing the identity of the tire/transponder in multiple transponder systems. In most cases, such transponders require battery power.
In some instances, a transponder may be implemented as an integrated circuit (IC) chip. Typically, the IC chip and other components are mounted and/or connected to a substrate such as a printed circuit board (PCB).
Some proposed systems have relatively complex transponder-sensor capabilities, including measurement and reporting of tire rotations and speed, along with tire ID, temperature, and pressure. For example: U.S. Pat. No. 5,562,787 (Koch, et al.; 1996), and U.S. Pat. No. 5,731,754 (Lee, Jr., et al.; 1998), incorporated in their entirety by reference herein.
Transponder Environmental Considerations
The environment within which a tire-mounted transponder must reliably operate, including during manufacture and in use, presents numerous challenges to the successful operation of the transducer. For example, the sensors (e.g., pressure, temperature) used with the transponder preferably will have an operating temperature range of up to 125° C., and should be able to withstand a manufacturing temperature of approximately 177° C. For truck tire applications, the pressure sensor must have an operating pressure range of from about 50 psi to about 120 psi (from about 345 kPa to about 827 kPa), and should be able to withstand pressure during manufacture of the tire of up to about 400 psi (about 2759 kPa). The accuracy, including the sum of all contributors to its inaccuracy, should be on the order of plus or minus 3% of full scale. Repeatability and stability of the pressure signal should fall within a specified accuracy range.
However it is implemented, a tire transponder (tag) must therefore be able to operate reliably despite a wide range of pressures and temperatures. Additionally, a tire transponder must be able to withstand significant mechanical shocks such as may be encountered when a vehicle drives over a speed bump or a pothole.
A device which can be used to indicate if a transponder or the tire has been exposed to excessive, potentially damaging temperatures is the “MTMS” device or Maximum Temperature Memory Switch developed by, Prof. Mehran Mehregany of Case Western Reserve University. It is a micro-machined silicon device that switches to a closed state at a certain high-temperature point. The sensor switches from an “open” high resistance state of, for example, over 1 mega-ohm to a “closed” low resistance state of, for example, less than 100 ohm.
Although it is generally well known to use pressure transducers in pneumatic tires, in association with electronic circuitry for transmitting pressure data, these pressure-data systems for tires have been plagued by difficulties inherent in the tire environment. Such difficulties include effectively and reliably coupling RF signals into and out of the tire, the rugged use the tire and electronic components are subjected to, as well as the possibility of deleterious effects on the tire from incorporation of the pressure transducer and electronics in a tire/wheel system. In the context of “passive” RF transponders which are powered by an external reader/interrogator, another problem is generating predictable and stable voltage levels within the transponder so that the circuitry within the transponder can perform to its design specification.
Suitable pressure transducers for use with a tire-mounted transponder include:
(a) piezoelectric transducers;
(b) piezoresistive devices, such as are disclosed in U.S. Pat. No. 3,893,228 (George, et al.; 1975) and in U.S. Pat. No. 4,317,216 (Gragg, Jr.; 1982);
(c) silicon capacitive pressure transducers, such as are disclosed in U.S. Pat. No. 4,701,826 (Mikkor; 1987), U.S. Pat. No. 5,528,452 (Ko; 1996), U.S. Pat. No. 5,706,565 (Sparks, et al.; 1998), and PCT/US99/16140 (Ko, et al.; filed Jul. 7, 1999);
(d) devices formed of a variable-conductive laminate of conductance ink; and
(e) devices formed of a variable-conductance elastomeric composition.
The Effect of Temperature on Gas Pressure
In a broad sense, for a mass of any gas in a state of thermal equilibrium, pressure P, temperature T, and volume V can readily be measured. For low enough values of the density, experiment shows that (1) for a given mass of gas held at a constant temperature, the pressure is inversely proportional to the volume (Boyle's law), and (2) for a given mass of gas held at a constant pressure, the volume is directly proportional to the temperature (law of Charles and Gay-Lussac). This leads to the “equation of state” of an ideal gas, or the “ideal gas law”:                PV=μRT        where:        μ is the mass of the gas in moles; and        R is a constant associated with the gas.        
Thus, for a contained (fixed) volume of gas, such as air contained within a pneumatic tire, an increase in temperature (T) will manifest itself as an increase in pressure (P).
Because of the ideal gas law relationship, it is recognized that in the context of pneumatic tires, one problem that arises during operation of tire pressure sensors of any kind is that tires heat up as they are run for longer periods of time. When a tire heats up, air which is confined within the essentially constant and closed volume of the tire expands, thus causing increased pressure within the tire, though the overall amount of air within the tire remains the same. Since the pressure nominally is different, a tire pressure sensor can provide different pressure readings when a tire is hot than would be the case if the tire were cold. This is why tire and vehicle manufacturers recommend that owners check their tire pressure when the tire is cold. Of course, with a remote tire pressure sensor, an operator may receive a continuous indication of tire pressure within the vehicle, but the indication may be inaccurate because of the temperature change. Thus, it is necessary to compensate for changes in temperature of the inflating medium (“gas” or air) within the pneumatic tire.
Patents dealing in one way or another with gas law effects in pneumatic tires include:
U.S. Pat. No. 3,596,509 (Raffelli; 1971), U.S. Pat. No. 4,335,283 (Migrin; 1982), U.S. Pat. No. 4,126,772 (Pappas, et al.; 1978), U.S. Pat. No. 4,909,074 (Gerresheim, et al.; 1990), U.S. Pat. No. 5,050,110 (Rott; 1991), U.S. Pat. No. 5,230,243 (Reinecke; 1993), U.S. Pat. No. 4,966,034 (Bock, et al.; 1990), U.S. Pat. No. 5,140,851 (Hettrich, et al.; 1992), U.S. Pat. No. 4,567,459 (Folger, et al.; 1986), all of which are incorporated in their entirety by reference herein.
U.S. Pat. No. 4,893,110 (Hebert; 1990), incorporated in its entirety by reference herein, discloses a tire monitoring device using pressure and temperature measurements to detect anomalies. As mentioned therein, a ratio of temperature and pressure provides a first approximation of a number of moles of gas in the tire, which should remain constant barring a leak of inflation fluid from the tire. (column 1, lines 18-26). More particularly, on each wheel are installed sensors (4) for pressure and sensors (6) for temperature of the tire, as well as elements (8 and 10) for transmitting the measured values as coded signals to a computer (12) on board the vehicle, such as disclosed in the aforementioned U.S. Pat. No. 4,703,650. The computer processes the measured values for pressure and temperature for each tire, and estimates for the pressure/temperature ratio (P/T estimate) are calculated for each wheel. Generally, the ratio for one of the tires is compared with the ratio for at least another one of the tires, and an alarm is output when a result (N) of the comparison deviates from a predetermined range of values.
Techniques for Transmitting Pressure and Temperature Readings from a Tire
Given that pressure and temperature conditions within a pneumatic tire can both be measured, various techniques have been proposed to transmit signals indicative of the measured pressure and temperature conditions to an external interrogator/receiver. For example, the following patents are incorporated in their entirety by reference herein:                transmit the signals individually, distinguished by phase displacements: U.S. Pat. No. 4,174,515 (Marzolf; 1979);        multiplex the signals: U.S. Pat. No. 5,285,189 (Nowicki, et al.; 1994), U.S. Pat. No. 5,297,424 (Sackett; 1994);        encoding the signals as separate segments of a data word: U.S. Pat. No. 5,231,872 (Bowler, et al.; 1993), and U.S. Pat. No. 4,695,823 (Vernon; 1987) which also incorporates both the telemetry and the pressure and/or temperature sensors on the same integrated circuit chip;        transmission between coils mounted on the wheel and on the vehicle: U.S. Pat. No. 4,567,459 (Folger, et al.; 1986);        use a frequency-shift key (FSK) signal: U.S. Pat. No. 5,228,337 (Sharpe, et al.; 1993);        backscatter-modulate the RF signal from the interrogator with the tire condition parameter data from the sensors, then return the backscatter modulated signal to the interrogator: U.S. Pat. No. 5,731,754 (Lee, Jr., et al.; 1998).        
U.S. Pat. No. 4,703,650 (Dosjoub, et al.; 1987), incorporated in its entirety by reference herein, discloses a circuit for coding the value of two variables measured in a tire, and a device for monitoring tires employing such a circuit. The coding circuit comprises an astable multivibrator which transforms the measurement of the variables, for instance pressure and temperature, into a time measurement. The astable multivibrator delivers a pulse signal whose pulse width is a function of the temperature and the cyclic ratio of which is a function of the pressure.
U.S. Pat. No. 5,054,315 (Dosjoub; 1991), incorporated in its entirety by reference herein, discloses a technique for coding the value of several quantities measured in a tire. As disclosed therein:                “Coding of the value of any number of quantities measured in a tire, for example its pressure and its temperature, is carried out using a ratio of time intervals TP/Tr, Tt/Tr. This frees the device from the effect of the time shift of the modulation system, the time shift affecting simultaneously the numerator and the denominator of said ratio.” (Abstract)        